Occluding vision can be counter-productive for helping people understand about blindness (as explained below).
However, for understanding the specific features and functional implications of various visual impairments, I have repeatedly found that visual simulators are worth more than a thousand words of explanation!
For less than $10 a pair, you can make your own vision simulators but, before I explain how to make them, I'd like to share with you some definitions of terms that I will use, courtesy of Dr. Duane Geruschat, Coordinator of Research at The Maryland School for the Blind. He says that...

"a scotoma is defined as an area of decreased perception surrounded by an area more sensitive to the same stimulus (Anderson 1987). There are two types of scotomas: absolute, and relative. An absolute scotoma is a defect which persists to the brightest and largest target. A relative scotoma is visible to some but not all targets. Thus, functionally, if the object being viewed, such as a face, sign, or building, is either large enough or bright enough, you may be able to see it with a relative scotoma. But with an absolute scotoma, assuming the target fits within the diameter of the scotoma, you will not see it.

"There is also a concept known as a depression, which is defined as an area of decreased sensitivity. This is also known as a diffuse loss. In a generalized depression, the visual field will have reduced sensitivity to light everywhere in the visual field. This is commonly seen in glaucoma prior to the loss of actual visual field. During a clinical visual field test, the person will respond but only when the light intensity or target size will be brighter or larger than normal.

"The zone of uncertainty is the effect seen when you change testing conditions. There are two principle ways of presenting a stimulus, static or dynamic. The traditional tangent screen test, which involves the movement of an isopter attached to a black wand, is an example of a dynamic (also called kinetic) visual field test. Computerized visual fields, which have a fixed number of lights which are programmed to illuminate, is an example of a static test. Testing with a moving versus fixed target will typically provide a slightly larger visual field than with a static target. Where the target is presented will also affect the apparent size of the visual field. For example, if you move from an area of sight to blindness, the subject will respond with a slightly larger visual field than when you present the target from non-seeing to seeing.

"My experience with practitioners is that they recognize that visual acuity fluctuates based on environmental conditions, which explains the limited relationship between clinical and functional acuity. What I haven't always seen practitioners appreciate is that clinical visual fields have the same type of problem. In school, some of us learned that visual fields were like a picket fence, suggesting that there is a sight / no-sight line. What you can now understand with the concepts of relative scotoma, depression, and an appreciation of the testing conditions, is that visual fields actually are affected by test conditions. Differing responses based on test conditions are acknowledged as the zone of uncertainty.

"What can make these concepts useful to practitioners is to appreciate that visual fields do and will fluctuate, just like acuity can and will fluctuate."

The citations for Duane's information are:

Anderson, D.R. (1987) Perimetry, with and without automation. C.V. Mosby, St. Louis, Missouri.

Now that we have a better understanding of what we're trying to simulate, thanks to Duane, I will describe how I make the simulators.
To make your set, buy welding goggles with screw-in glass inserts (not the kind with a solid front).
You can get "cup goggles" (with a flexible part over the nose), but for a few dollars more I prefer the "cover-all" goggles that can fit over glasses and are more comfortable because if I take off my glasses to put on a simulator, I am already starting with 20/300 vision.
Sources of welding goggles:

Both sets of goggles used to come with two clear glass lens inserts and two of dark plastic and you could purchase extra lenses as needed, but by 2013 this is no longer true.
Sellstrom only provides one set of lenses for each set of goggles you purchase -- the "cup goggles" can be ordered with clear lenses but the "cover all" goggles no longer come with clear lens, though they can be ordered separately (part #04501) but one set of lenses costs more than the goggles themselves do!
Katie Belmont found coverall googles with clear lenses at Amazon.
Thanks Katie, this is great to know! She said:

These goggles seem just fine for this purpose.
The black inserts can be removed if necessary and they have flip down covers so you could even flip them up for one type of simulation and then flip them down for another type.
They also fit well over glasses.
The clear lenses are plastic not glass, but since we're not welding and are just going to ruin the lenses anyway, I don't think that's a big deal!
They have air vents in the sides that allow a bit of visibility but you could just take out the little filter insert and put a little piece of black electrical tape over it.

The prices for the goggles at Amazon varied incredibly -- Katie got her goggles for less than $2 in March 2011 but two years later the goggles were $4.70 each and within 48 hours they had gone to $6.75 with a notice that there were only 5 left in stock!
Perhaps when they restock the price will go down again?
Comments on Amazon mentioned great discomfort with the nosepiece digging in, which was relieved by padding it with felt or other material.

Once you have your welding goggles, make circles from foamboard or heavy cardboard to fit into the goggle for an occluder. You may need one occluder for such visual impairments as retinitis pigmentosa and macular degeneration because it's impossible to simulate those conditions with both eyes at once -- if you get the scotoma placed just right for both eyes to focus on a close object, they will be mis-aligned when focusing on a far object and vice versa. The goggles with both eyes totally occluded with circles makes a blindfold that some people find more comfortable and sanitary than fabric blindfolds. Some frame shops will give you scraps of foamboard -- you can cut the circles with a good knife.

Following are instructions to make inserts for different effects:

Macular Degeneration: Have someone look through the goggles with a glass insert, and mark where the center of vision falls. Place a thick dot of clear nail polish over the part that the person will be looking through (I make my scotomas fairly large so that the person doesn't have to work so hard to avoid cheating; for the cover-all goggles, my scotomas are about a half inch in diameter but for the cup goggles, where the glass is closer to the eye, the scotoma can be smaller).

When the nail polish has dried to the sticky stage, dab it with a tissue (don't let the tissue drag the polish and your scotoma all over the glass!). For a "depression" one dab'll do ya, but for a relative or absolute scotoma, keep dabbing every minute or so until it is as opaque as you want.

Retinitis Pigmentosa or Glaucoma: You can either make the field loss absolute by using funnels or foamboard or thick cardboard, or relative by using clear nail polish on a clear glass insert. To make it from foamboard, take one of the occluder circles and drill out a hole a little to one side of center. Try to avoid getting the "pinhole" effect -- an occluder with a tiny hole held close to the eye will diffuse the light and, for some people, will give them a more clear image than they normally have. To avoid this, make the hole larger (to simulate a small field without the pinhole effect, you can put a funnel into the goggles, so that the hole is further from the eye).

To make the field loss using clear glass and nail polish, place the nail polish all over the glass but leave a clean circle near the middle where the person will be seeing straight ahead. Proceed as you did for the macular degeneration, dabbing it with tissue until you have the desired effect.

Night blindness from Retinitis Pigmentosa (RP): If you can find an area with just the right amount of darkness, you can dramatically illustrate night blindness using a foamboard RP simulator and one of the dark glass inserts that comes with the goggles (the inserts that come with some goggles are too dark -- mine were shade 5.0). Try this in different areas until you have found an area where it is exactly dark enough to be effective. Have people put on the goggles with only one occluder. In the opening over the remaining eye, first have them look through the dark lens, then remove it and have them look through an RP simulator. In both situations, they should still be able to see many details in the room. Then, with the RP simulator still on, place the dark lens over it, and if you have chosen a good place with exactly the right darkness, they will either see nothing, or they will see much less detail than they did with just the RP simulator or the dark lens alone. Explain that with the dark lens and a full field, the eye can gather enough light to see, and with constricted field and enough light, the eye can still see, but with both a restricted visual field and reduced light, the effects are not added, they are multiplied.

Cataract: Take a clear glass insert and spray with clear varnish or nail polish, and dab with a kleenex when sticky; repeat until it is as opaque as you want it. You may want to make part of it thicker, leaving part of it relatively clear. Another suggestion is to use a yellow clear plastic sheet over wax paper.

When you've made your simulator set, you can have someone with normal vision wear them while looking at eye charts to measure the acuity.
For the visual field loss simulators, place a yardstick or some paper on the floor or wall where you can mark it.
Put on the goggles, and look at the target from exactly three feet away (I place the end of another yardstick on the target and bring my face and goggles up to the other end of the yardstick to be sure I'm looking at it from three feet away).
Note how wide the field of vision on the target is, and measure it.
From a yard away, seeing a little less than 4 inches across is 6 degrees of visual field; 6 inches across is 9.5 degrees, 8 inches across is 12.5 degrees, and 12.6 inches is 20 degrees of visual field (ask someone who knows geometry, like my son Mark, to help you figure this out!).

I mark the degrees right on the simulator.
The degree of visual field that is visible, however, will change if you put your simulator in a different kind of goggle (that is, a hole placed close to the eye in the flexible cup goggles will give you a larger visual field than the same hole placed further from the eye in the cover-all goggles).

Students in the Fall, 2016 graphic design class at North Carolina State University made their own simulators -- some of them were modeled here!

Dr. Laura Bozeman, Associate Professor/Director of Vision Studies at
UMass Boston, did her dissertation on fidelity in low vision simulation. She says that people in technical areas, such as engineering and medical fields, have used simulation for a long time to teach and train, and they do extensive research on the effectiveness of simulation. They have found three, crucial components to simulator design and use:

1) One must know the purpose of the simulator (what do you want to achieve with it?)
2) What fidelity is needed for the simulator to accurately represent its real life counterpart?
3) How does one assess the simulator?

When we apply these components to low vision, we need to consider:
1) what is the purpose of the simulation. For example, is it clinical, such as, do we want to teach a person how to administer a visual acuity test? If so, then clinical fidelity is needed the simulator should elicit those responses of reduced visual acuity, etc. in a static, clinical environment. Or is it a functional outcome? For example, do we want to simulate errors that are typically seen in dynamic situations by individuals who have essential central scotomas, such as problems with reading street signs, recognizing faces, and detecting the traffic?

2) How do you design the simulator so that normally-sighted folks demonstrate these behaviors in a functional setting? For example, clinically it is easy to not "look around the scotoma", but functionally it is difficult not to "cheat." Thus, for functional purposes Laura uses a simulator that is almost completely occluded except for a small area around the periphery, because it allows you to move about, but you cannot use your central vision along the periphery. Clinically, this particular simulator is a wash because you cannot see anything centrally, but remember, it was designed with functional fidelity in mind to elicit the specific, functional characteristics exhibited by people who have low vision.

3) Consequently, the clinical simulators should be assessed clinically and the functional simulators should be assessed via functional observation. For observations of low vision simulations of functional orientation and mobility situations, Laura uses a form based on Geruschat and D'lune's "Critical Incidents Tally Sheet" that tallies stumbles, bumps, drop off errors, orientation errors, street crossing errors, as well as the time required to travel the routes.

Much of the literature review (outside of the vision profession) for Laura's dissertation is based on:

Su, Y.D. (1984). "A review of the literature on training simulators: Transfer of training and simulator fidelity" (published in 1984 as Technical Report 84 1; Arlington, VA by the Personnel and Training Research Programs office of Naval Research.

As Laura said, it is important to consider the purpose of your simulation in order to prepare your "simulators." Several years ago, some visually impaired students asked Susan Spicknall (we knew her as Susan Herron) and me to give an inservice to their university professors who, they felt, were not being understanding or cooperative about their needs.

Susan and I quickly agreed that blindfolding them would not achieve our purpose because it would simply frighten them and distract their attention from the points we wanted them to get. So we decided on a different "simulation" that would get at the heart of the matter, which was lack of access to the classroom and their instruction, not dealing with the fear and frustration of recent blindness.

We opened the inservice by greeting them and distributing our handouts (unreadable copies of smeared print and scribbles). We referred to the handouts ("Let's discuss the second item on your list...") without letting them know what was on it. We then explained the various visual impairments using slides -- but the projector was projecting blank screens. We tantalized them by saying things like, "Retinitis pigmentosa and macular degeneration are the visual impairments that are probably least understood by people, as will be clear when you see the slides of what people with those visual impairments can see" (click -- another blank slide with more comments that are tantalizing but not revealing about what is on the slide -- "isn't that interesting? You can see why someone with this vision could read a newspaper but miss seeing a fire hydrant and trip over it."). Then we list all the visual impairments that are important to know on the board -- in illegible scribbles of course -- and again refer to the list on the board but without saying what they are: "This one (pointing to a scribble) is the most common cause of blindness in the United States, but this one (pointing again) is the world-wide most common cause of blindness, and you can understand why."

We were afraid these dignified professors would get angry or insulted at our mistreatment of them, but they took it very well (we were grinning mischievously as we went through our presentation) and at the end they laughed good-naturedly. We then went through the entire presentation again (this "presentation" was only about 5 minutes long), again using illegible scribbles and blank slides, but this time we provided access to the information that was supposedly there. When we referred to the blank handouts, we read it ("The third item in your handout, macular degeneration, is confusing to people because, as you can see in this slide, the center of the picture is obliterated, just as the center of these people's vision is obliterated. You can see that the peripheral vision, or "side vision", is fuzzy. I will list the visual impairments on the board -- first, there is diabetic retinopathy (scribble scribble)" and we read the words on the board again whenever we pointed to them.

The students were extremely pleased with our inservice, and felt that it went a long way to sensitizing their professors to their situation.

Simulating Blindness while Walking SafelyApril, 2010

What can you do when someone who doesn't know how to use a cane wants to experience movement or walking without vision?
For example, a student teacher of blind children wanted to experience problem-solving and the challenge of staying oriented while traveling blindfolded in his neighborhood, and the manager of a program for elderly blind people wanted to learn how people can stay oriented around the building without vision.

Is it possible for such people to walk around with a blindfold under supervision independently and yet be safe and feel confident?
It normally takes considerable time to become proficient enough with a cane to reliably avoid obstacles and notice steps and curbs (see Stages of Learning to Use a Cane), so can people simulate traveling without vision safely without first preparing with extensive cane training?

Yes they can, if they use an Alternative Mobility Device (AMD)!
After learning to notice when the AMD drops over an edge or bumps into an obstacle, they can avoid tripping and falling while walking blindfolded.
This allows them to walk without any guiding or even any contact from others, although they should be monitored for safety when they are near hazardous areas which require good non-visual skills to navigate, such as streets.

For example, the photos below show a group of us studying echolocation -- Mike, who is blind, Hiromi, an O&M student from Japan, my son Stephan, and me.
For this particular session, Mike led us to a site where we could practice using echolocation to distinguish bushes and walls.
It was important that we not be able to see the site beforehand, so we all followed Mike with our eyes closed -- Hiromi and I used a cane and Stephan, who had no cane skills, was nevertheless able to walk with the rest of us safely with an AMD.
The last photo shows the four of us noticing the sound at a wall with an overhang. [Photos courtesy of Liz Aldridge]

Simulated blindness can have negative effects on people's perceptions of visually impairedJanuary, 2015

When doing inservices about blindness to lay people, I usually avoid blindfolding them because often, whatever insights they gain about the use of other senses is more than offset by feeling frightened by the experience.
It is with little surprise, therefore, that I learned that a report from a study
by Arielle Silverman, Jason Guinn and Leaf Van Boven at the University of Colorado Boulder indicates that wearing a blindfold while performing everyday tasks has negative effects on people's perceptions of blindness.

Compared with study participants who weren't blindfolded, participants who were blindfolded for activities were more likely to believe that people who are blind are less capable of work and independent living, and said they would be less capable if they personally became blind and slower to adjust to their new world.
There is a blog about this study with some wonderful insights at VisionAware.

UPDATE May 2017:

Research from Hiram College indicating that simulation or role-playing of a disability can promote distress, discomfort and disinterest is reported in the April 11, 2017 edition of ScienceDaily.